A conventional rough paper-adaptable thermal transfer recording medium uses the so-called bridgingly transferable heat-meltable ink layer which is transferred as bridging over depressed portions of a rough paper while adhering to only protruded portions thereof. In order to impart such bridging transferability to an ink layer, a heat-meltable material having a high melt viscosity such as a high melt viscosity resin, for example, ethylene-vinyl acetate copolymer, is used as a main component of the vehicle thereof.
However, such an ink layer having a high melt viscosity is poor in fixing property and, hence, involves drawbacks such as falling-off of print images obtained therefrom due to abrasion or the like. In order to improve the fixing property of such an ink layer, a large amount of energy must be applied during thermal transfer.
A heat-meltable material having a high melt viscosity shows a small viscosity change over the temperature range of temperatures in the vicinity of ordinary temperature to transferring temperature due to the characteristics of the material.
FIG. 1 is a graph schematically showing relationships between temperature and melt viscosity with respect to various heat-meltable materials. Curve B shows a viscosity curve of an ethylene-vinyl acetate copolymer having a high melt viscosity.
As described above, a heat-meltable material having a high melt viscosity shows a small viscosity change and, hence, when it is attempted to lower the viscosity of the material at a temperature for transferring in order to improve fixing property of print images, the material shows a low viscosity even at temperatures in the vicinity of ordinary temperature, as shown in Curve C of FIG. 1.
Accordingly, when an ink layer is adapted to be transferred with a small amount of energy, the ink layer exhibits poor bridging transferability and also has a decreased softening point, resulting in the following drawback. When printing is performed continuously, heat is accumulated in a thermal head to heat the overall head. Since the softening point of the ink layer is low, portions of the ink layer which correspond to non-activated heat generating dots are also softened, resulting in scumming of a receptor paper.
In view of the foregoing, an object of the present invention is to provide a thermal transfer recording medium which exhibits good bridging transferability and fixing property and has a high softening temperature, thereby forming clear print images even on a rough paper with a small amount of energy and causing no scumming of a receptor paper even in continuous printing.